So, the conclusion is the development of ectoderm, mesoderm and endoderm eh?? Oh, one more thing..let's say that some animals only have ectoderm and endoderm. So, do organs that developed from mesoderm can still be replaced by those two??

Q: Why are chemists great for solving problems?
A: They have all the solutions.

victor wrote:So, the conclusion is the development of ectoderm, mesoderm and endoderm eh?? Oh, one more thing..let's say that some animals only have ectoderm and endoderm. So, do organs that developed from mesoderm can still be replaced by those two??

this phenomenon is called gastrulation, not neurulation. Gastrulation takes place when the primitive streak occurs under the influence of beta - catenin and members of the TGF-beta-family.